Yes it is using the scenario builder.
Create a custom scenario with all races enabled, including the dread lords if you like, set yourself as the custom race and there you go. Just start the game as normal and select that scenario.
I should point out you need to set up the morality/skill level of each race, look in the xml files for this, it isn't hard to do

Also 11 never survive, races often surrender so I have had no slowdown in my experience.
It may crash in the UP though, i've never had them all in at once, one always dies off
Also enable cheats and press control + alt + D a few times, this will spawn minor races to unused 0 quality planets, I think there are five or six, always the same ones.